<p> Load a bunch of frames containing videos.
<script>
var url = new URL(location);
var urls  = url.searchParams.get('urls').split(',');
var numFrames = urls.length;
var numSubresources = parseInt(url.searchParams.get('numSubresources'), 10);
function createFrames() {
  // Wait for |numFrame| messages. Each message is received when a given frame
  // finishes (errors/completes) all of its subresources.
  var numMessages = 0;
  var completePromise = new Promise(resolve => {
    window.addEventListener('message', function(e) {
      numMessages++;
      if (numMessages == numFrames)
        resolve(true);
    });
  });

  for (var i = 0; i < numFrames; i++) {
    var frame = document.createElement('iframe');
    frame.src =
        "/cross-site/example.com/site_isolation/subframe_resources.html?url=" +
        urls[i] + "&numSubresources=" + numSubresources;
    document.body.appendChild(frame);
  }

  return completePromise;
}

</script>
